The objective of this project is to study combinations of anticancer agents from a variety of perspectives in order to increase the clinical effectiveness of cancer therapy. Experimental leukemia and solid tumor models as well as normal hematopoietic stem cells are the cell populations to be studied. Both in vivo and in vitro analysis will be done. I will specifically examine the following combinations in this proposal: 1) Thiols including WR-2721 in combination with alkylating agents. The finding of potentiation of drug cytotoxicity will be examined in detail with different thiols, different alkylating agents and different tumors. These cellular studies will be followed by molecular studies, initially alkaline elution methodology, to determine the mechanism for this interaction. 2) Radiation plus either 5-fluorouracil (FUra) or Actinomycin D (Act-D) will be studied specifically for the importance of dose (including schedule), sequence and interval for the synergistic effects noted. Mechanism studies will be directed towards defining whether modification of DNA damage or repair is affected by the drugs. 3) Three specific drug combinations will be examined. Each has previously demonstrated synergistic interactions and here we will define the effect further and study possible mechanisms for the interactions. The combinations are cytosine arabinoside plus Daunorubicin, FUra plus one or more alkylating agents and Cyclophosphamide plus nitrosoureas. Flow cytometry will be used to define any synchronization basis for the synergy and both alkaline elution and drug uptake will be employed to define possible biochemical bases.